Although I'm not the developer of either the Shackleton or the Belfast, I do develop others, so I feel I can reply.
Because such aircraft are developed in our spare time, as a hobby, nobody likes to promise any release dates etc which can't be honoured.
Similarly, the aircraft modelled are very much a matter of whether the developer really wants to build one, considering the hundreds of hours these take. So it's rare for requests to be undertaken.
However - that's just my personal view so you never know, you might get lucky.
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